The present invention relates to a process for preparing perovskite-type compound powders used as raw materials of ceramics.
In recent years, with the production of miniaturized light-weight high performance electronic devices there has been a demand to form thin films or to miniaturize ceramics of perovskite-type compounds used in capacitors, thermisters, etc. for the devices. Consequently, the formation of thin films and miniaturization have been investigated from the viewpoints of techniques for the production of ceramics such as formulation, forming and firing.
However, perovskite-type compounds which have been used as the raw materials are those obtained by a solid phase reaction and the average particle size thereof is at least 0.8 .mu.m. Accordingly, even if techniques for the production of skillfully ceramics are skillfully used, the obtained ceramics have a limit in achievable thin film formation and miniaturization, and it has not been possible to achieve sufficient thin film and miniaturation.
That is to say, since the conventionally used perovskite-type compounds are prepared by admixing a carbonate or oxide of at least one member selected from metal elements (hereinafter referred to as group A elements) such as Mg, Ca, Sr, Ba and Pb with an oxide of at least one member selected from metal elements (hereinafter referred to as gruop B elements) such as Ti, Zr, Hf and Sn, and calcining the mixture at a high temperature of not less than 1,000.degree. C. to produce perovskite-type compounds, and thereafter mechanically pulverizing the compounds by a ball mill or the like, the particulate perovskite-type compounds having an average particle size of not less than 0.8 .mu.m have only been obtained. Consequently, as mentioned above, ceramics molded using them as the raw material have possessed the problem that miniaturization and thin film formation cannot be sufficiently achieved.
In order to solve such a problem, in Japanese Patent Publication Kokai No. 59-39726, No. 61-91016, No. 60-90825 and No. 61-31345 there is proposed a process for preparing a fine particulate perovskite-type compound having an average particle size of at most 0.2 .mu.m by a wet process.
However, though the wet process can provide fine powders of perovskite-type compounds, it has the defect that the obtained products are poor in crystallinity because the reaction does not sufficiently proceed as compared with perovskite-type compounds obtained by a solid phase reaction and also because the products contain a large quantity of water in the crystal structure.
Accordingly, the fine particulate perovskite-type compounds obtained by the above-mentioned wet process have the defect that if they are dispersed and incorporated with a binder in an aqueous system when calcining them at a temperature at which no growth of particles occurs and used as a raw material for ceramic thin films, a water-soluble component deposits in the forming and drying steps, and the thus obtained ceramics have a nonuniform composition and show a large scatter in physical properties and electric properties.
Also, although it is possible in an oil system to disperse the fine perovskite-type compound powder obtained by the above-mentioned wet process and additives, it is difficult to sufficiently control the reaction of the perovskite-type compound and the additives at the time of firing, and the ceramics after firing show a scatter in physical and electric properties like those obtained by dispersing in an aqueous system because the degree of the progress of reaction of the perovskite-type compound is low and because the crystallinity is insufficient.
In order to eliminate the above defect, the crystallinity may be improved by raising the calcination temperature so as to make the reaction progress sufficiently. However, if the calcination temperature is raised, the growth of particles takes place and the particles characteristics as fine particles are lost, thus the products become those similar to perovskite-type compounds obtained by the solid phase reaction and cannot be used for thin film formation and miniaturization of ceramics.
Perovskite-type compounds having a large average particle size obtained by the solid phase reaction must be finely divided by mechanical pulverization. When they are pulverized, introduction of impurities from the pulverization media is unavoidable. Since these contaminants cannot be separated, they are an obstacle to forming thin films, ceramics and controlling the electric properties of capacitors.
Also, the powder obtained by pulverization is wide in its distribution of particle sizes and is poor crystal form (broken). Accordingly, when it is used as electro-ceramics, it is difficult to control the electric properties of devices.
In particular, barium titanate has been commonly used as the raw material for electro-ceramics among perovskite-type compounds. Barium titanate obtained by the above-mentioned solid phase reaction has a tetragonal crystal form, and barium titanate obtained by the above-mentioned wet process has pseudo-cubic crystal form. In order to obtain ceramic thin films having high performances, it is desirable that the barium titanate is tetragonal crystals and has a small average particle size for the purposes of achieving dense fine grained ceramic films. However, while wet process provides barium titanate having a small average diameter, it has the disadvantages as mentioned above. On the other hand, barium titanate obtained by the solid phase reaction is tetragonal crystals, but it has the disadvantage of requiring a pulverization operation because of its large average diameter. Moreover, the pulverized barium titanate contains impurities and has a wide distribution of particle size. No particles having a narrow distribution of particle size are obtained by pulverization. Further, if the barium titanate is finely pulverized to 0.3 .mu.m or less, the crystal form changes from tetragonal form to cubic or amorphous form.
Barium titante obtain from the solid phase reaction originally shows tetragonal crystal form at room temperature and causes crystal transition to cubic crystal form at the Curie point in the vicinity of 120.degree. C.
Conventional barium titanate particles obtained by the solid phase reaction having a tetragonal crystal form, undergo crystal transition to cubic crystal form by heating to more than the Curie point. However, barium titanate obtained by a low temperature wet process shows cubic crystal form at room temperature. This cubic barium titanate obtained by the wet process has a longer crystal axial length compared to the crystal axial length of the cubic crystal form shown by tetragonal barium titanate at a temperature above the Curie point when the both are compared at a temperature above the Curie point and, therefore, barium titanate which shows cubic crystal form at room temperature is often expressed as pseudo-cubic crystals for distinguishing between the two forms.
In case of the pseudo-cubic barium titanate, the axial length decreases as the calcination proceeds, and finally reaches the axial length that cubic barium titanate should have originally. The pseudo-cubic barium titanate that has decreased its crystal axial length by calcination to the short crystal axial length, shows tetragonal crystal form when cooled to room temperature below the Curie point.
The unit cell of tetragonal barium titanate has axial lengths (a, a, c), and it is recognized that the ratio of lattice constants, namely the c/a ratio, is 1.01.
Barium titanate whose crystal axial length has not been sufficiently decreased by calcination, does not change into tetragonal crystals when cooled to room temperature below the Curie point, and it remains as it was (cubic form). When calcining at a temperature of 700.degree. to 900.degree. C., there are cases where the calcined products show a clear difference between the axial length and the axial length as if they are tetragonal crystals. However, the c/a ratio of such products is not more than 1.008, and they do not show the presence of definite Curie point when observed by differential thermal analysis (DTA) even if heated above the Curie point.
In other words, exactly speaking, barium titanate which shows pseudo-cubic crystal form at room temperature and barium titanate whose c/a ratio is not more than 1.008 are distinct from tetragonal barium titanate as obtained by the solid phase process, the c/a ratio of which is not less than 1.009. The values for the c/a ratio of commercially available barium titanates prepared by a solid phase process calculated by rounding to three decimals the c/a ratio measured from X-ray diffraction peaks for (200) and (002) planes are not less than 1.009, and most of them are 1.010.
Since, as mentioned above, fine particles of perovskite-type compounds cannot be obtained by the solid phase reaction and since the conventional wet processes provide only perovskite-type compounds insufficient in the degree of the progress of reaction and the crystallinity, there has been a problem that ceramics having good characteristics cannot be obtained.
Accordingly, an object of the present invention is to provide a process according to which fine particulate perovskite-type compounds having a good crystallinity can be easily prepared.
Another object of the present invention is to provide a tetragonal barium titanate fine powder having average particle size of not more than 0.3 .mu.m and a good crystallinity, that the difference between the a axial length and the c axial length is definite, and the c/a ratio calculated from (002) and (200) peaks of X-ray diffraction pattern is not less than 1.009, and which definitely shows the Curie point observed by DTA.
These and other objects of the present invention will become apparent from the description hereinafter.
The term "pseudo-cubic barium titanate" as used herein means barium titanate whose X-ray diffraction pattern at room temperature shows cubic crystal form, and barium titanate for which the separation of X-ray diffraction peaks derived from (200) plane and (002) plane is indefinite because the c/a ratio is not more than 1.008.
With respect to other perovskite-type compounds which are cubic crystals at room temperature, in order to conform to the expression of the crystal system for barium titanate, those prepared by a wet process and poor in crystallinity due to a longer crystal axial length are also expressed as pseudo-cubic crystals in the specification.